Operational amplifiers have been used in the electronics industry for many years. These amplifiers are constructed using integrated circuit methods and are packaged in integrated circuit casings or in packages similar to those used for discrete transistors. They provide an inexpensive and convenient way to amplify signals with a minimum of discrete parts.
When amplifying small signals, care must be taken to minimize circuit noise. As the amplitude of the input signal to an amplifier is decreased, the amount of circuit noise that can be tolerated also decreases. For example, if the input signal voltage is 1 volt peak-to-peak, then a circuit noise level of 100 micro-volts (uV) is of little consequence. However if the input signal voltage is 50 uV, then a circuit noise level of 100 uV swamps out the desired signal making the amplifier useless. Therefore great care is taken to ensure that input voltage noise (V.sub.13 noise) of a small signal operational amplifier is minimized. But other devices also contribute to the circuit noise.
The source, biasing and feedback resistors associated with an amplifier each contribute to the circuit noise due to two phenomena. First there is "Johnson Noise" and second there is noise generated by the input current to the amplifier.
Due to Johnson Noise, each resistor generates a voltage noise equal to "sqrt(4 KTR)" volts per root Hertz (Hz). In the Johnson noise equation, "K" is Boltzman's constant, "T" is the temperature in degrees Kelvin and "R" is the resistance in ohms.
The inputs of an amplifier produce a noise current (I.sub.13 noise). As this noise current flows through resistors attached to the amplifier inputs, a noise voltage is generated across the resistors. The magnitude of the noise voltage is (I.sub.13 noise*R). The noise generated across the input biasing resistor of an inverting amplifier can be a significant or even dominant noise source for the amplifier.
FIG. 1 illustrates a prior art small signal operational amplifier. The operational amplifier 101 has a non-inverting input 103 and an inverting input 105. A biasing resistor "R3" 107 is connected in series between the non-inverting input 103 and a signal ground. A feedback resistor "R2" 109 is connected between the inverting input 105 and an output 111 of the operational amplifier 101. Connected in series between a voltage input source 113 and the inverting input 105 is an input resistor "R1" 115 and a coupling capacitor "C1" 117.
The inputs of an amplifier require an input bias current to ensure a zero direct current (DC) offset voltage at the amplifier output 111. In the example illustrated in FIG. 1, the inverting and non-inverting input bias currents of the amplifier are assumed to be matched. Resistors 107 and 109 are equal, 5000 ohms (5K), to ensure a zero DC offset at the amplifier output due to input bias currents.
The equivalent input noise voltage per root Hz of the circuit for a frequency (f) much greater than determined by the formula "f=1/2*3.142*C1*R1" is given by the formula: EQU sqrt (V.sub.13 noise.sup.2 +(I.sub.13 noise (R1*R2/(R1+R2))).sup.2 +4KT(R1*R2/(R1+R2))+(I.sub.13 noise*R3).sup.2 +4KT(R3))
Since R3 is greater than (R1*R2/(R1+R2)), then the noise contributions of R3 (107) exceed those of R1 (115) and R2 (109). If V.sub.13 noise is small then the noise contributions of the input biasing resistor R3 can be the dominant source of noise in the amplifier.
Two prior art methods have been used to reduce the noise contribution of the non-inverting input resistor. The first method reduces the non-inverting input biasing resistor value and the second method reduces the values of the input, feedback and non-inverting biasing resistor. Both methods have disadvantages.
In the first method, if the non-inverting input bias resistor R3 (107) is reduced, then both the Johnson and input current noise contributions related to R3 are reduced. However, if the value of the input biasing resistor R3 alone is reduced, then it's value will no longer match the DC impedance at the inverting input R2. When the inverting and non-inverting input impedances no longer match at DC, then the input bias currents of the amplifier are no longer compensated and a finite output voltage will result. This is an undesirable result as the output voltage 119 will no longer be zero for a zero input voltage (an output offset voltage is created).
In the second method, if all of the resistors R1, R2 and R3 are all reduced in value, then the input bias currents will still be balanced and the output of the amplifier will still be zero for a zero input voltage. However reducing the input resistor may not be desirable since the input impedance of the amplifier may be too low for the input voltage source.
Therefore what is needed in the industry is a method for reducing the noise contribution from the non-inverting bias resistor without decreasing the input impedance of the amplifier or creating an offset output voltage.